1. Field of the Invention
This invention pertains to the field of aircraft capable of vertical take-off or landing and in particular relates to improved stabilization of such aircraft in vertical flight.
2. State of the Prior Art
Much effort has been directed towards the development of aircraft capable of vertical take-off or landing which are not restricted to airport runways but can land and take-off from any relatively small open area.
Rotary wing aircraft, such as helicopters, are one answer to this problem. Although helicopters are capable of vertical flight and hover, they require large exposed rotor blades which are vulnerable to strikes and dangerous to persons in the vicinity of the aircraft on the ground. Further, helicopters achieve horizontal flight by cyclic control of the rotor blade pitch, and control ascent and descent by collective control of the blade pitch. The rotor is exposed to the airstream during horizontal flight so that large differences in airspeed over the blades occur as each blade advances into the airstream on one side of the rotor disk and then recedes on the opposite side of the disc where the blade moves with the airstream. The resultant difference in lift must also be compensated by cyclic adjustment of the rotor blades. These considerations lead to complex rotor control systems which are difficult and costly to maintain, and which require considerable pilot training and skill.
In order to overcome these difficulties, aircraft have been proposed which are capable of vertical flight for takeoff and landing, but which once airborne, transition to more efficient winged horizontal flight. These designs have power plants which develop vertical thrust only during take-off and landing, and provide horizontal thrust during forward flight. Various schemes have been devised for developing the vertical and horizontal thrust vectors, including rotary nozzles for jet engines, deflector vanes for propeller drives, and pivotably mounted engines, among others. One known approach to this type of aircraft is the use of a ducted fan or fans mounted in the airframe for developing vertical thrust aligned with the aircraft center of mass. Horizontal thrust is developed either by deflecting the vertical thrust once takeoff has been achieved, or by operating a separate horizontal thruster.
VTOL aircraft with a single vertical thruster, such as a ducted fan, present special stability problems during the transition between vertical and horizontal flight modes. In vertical flight close to the ground, the aircraft may be stabilized against wobbling by the cushion of pressurized air developed between the aircraft and the underlying ground surface. The same cushion of pressurized air, however, provides a zero friction support and allows the aircraft to move easily or skitter in a horizontal plane, a problem which is addressed below. This effect, however, is limited to close proximity to the ground surface, and diminishes rapidly with altitude of the aircraft. During horizontal flight on the other hand, aircraft attitude may be stabilized by conventional control surfaces on the airframe. An interval exists, however, where the aircraft's altitude no longer allows build up of a cushion of compressed air underneath, yet the horizontal airspeed of the aircraft is insufficient for effective use of the control surfaces. Some additional means must therefore be provided for stabilizing the aircraft during this interval. Adjustable thrust deflectors and multiple thrusters have been employed which continuously respond to and counteract deviations of the airframe from a reference attitude, but this approach is complex and difficult to achieve in practice.
A simpler approach relies upon inertial stabilization by exploiting the gyroscopic effect of a rotating disc or ring. One known expedient involves the use of a horizontal fan as a gyroscopic rotor to obtain both vertical thrust and horizontal stability of the aircraft, as exemplified by U.S. Pat. No. 4,773,618 issued to Ow, where lift is derived by directing jet exhaust gases over air foils in a large fan which also provides gyroscopic attitude control in all phases of flight. A somewhat different approach is described in Wright et al. U.S. Pat. No. 4,778,128 which shows a ducted fan driving a radial airflow over a single circular airfoil to provide lift, the air foil being rotatable for inertial stabilization of the craft.
De Toia, U.S. Pat. No. 4,050,652 shows an airframe with counterrotating discs which provide both lift and gyroscopic stability.
Messina U.S. Pat. No. 4,461,436; Jordan U.S. Pat. No. 4,387,867 and Bostan U.S. Pat. No. 4,312,483 all show disc shaped gyroscopically stabilized "flying saucer" type craft with a central ducted fan and a separate rotating disc which provides gyroscopic stability. Jordan and Messina drive the gyro disc with vanes in the ducted fan airstream, while Bostan provides a separate electric drive for the gyro rotor.
The prior art designs enumerated above fall into two groups: a first group comprising aircraft which have no aerodynamic control surfaces active during horizontal flight and therefore require gyroscopic stabilization in all phases of flight, and a second group of aircraft stabilized by means of aerodynamic surfaces during forward flight without resort to inertial stabilization. In the latter case, the known designs do not provide for a transition between gyroscopically stabilized flight and purely aerodynamically stabilized flight.
The prior designs rely on the gyro rotor not only to hold the aircraft in a constant plane but also to counteract the torque of the fan drive, which otherwise would tend to yaw the entire airframe in a direction opposite to the fan rotation. In such an arrangement the gyro rotor cannot be stopped in flight without transmitting the fan's reaction torque to the airframe.
It is desirable to disable the gyroscopic stabilization system during transition to a purely aerodynamically stabilized flight mode because the inertia produced by the gyro rotor interferes with aerodynamic control of the aircraft. The gyroscopic tendency to maintain a constant plane of rotation hinders, for example, the ability to bank the aircraft during turns in forward flight.
Yet another complication characteristic of vertical take-off and landing aircraft is the tendency of the aircraft to skitter on the cushion of compressed air created between the aircraft and the ground surface while hovering during take-off or landing in close proximity to the ground. Such aircraft tend to move unpredictably in any direction over the ground surface, and may respond to slight sloping of the ground surface, prevailing winds or any slight lateral bias in the vertical thrust. This problem cannot be overcome by gyroscopic stabilization of the aircraft since the skittish motion does not necessarily involve any tilting or wobble of the airframe, only motion in a horizontal plane parallel to the ground surface. Various schemes have been devised in an attempt to resolve this difficulty, including the use of lateral thrusters, swiveling nozzles arranged about the periphery of the aircraft and swiveling slat arrangements for deflecting a portion of the vertical thrust laterally to hold the aircraft against horizontal displacement during low altitude hover. However, no control system has been developed which is of sufficient simplicity, reliability and effectiveness for overcoming this problem.
A continuing need exists for vertical take-off and landing aircraft which are stable during vertical flight without hindrance to aerodynamically stabilized winged horizontal flight.